1. Field of the Invention
The present invention relates to a variable valve timing control apparatus for an internal combustion engine.
2. Description of Related Art
A variable valve timing device is mounted on an internal combustion engine of a vehicle. The variable valve timing device changes a valve open/close timing of an intake valve or an exhaust valve driven by a camshaft, so as to improve engine output, gas mileage and exhaust emission. A rotation phase of the camshaft is changed with respect to a crankshaft, thereby changing the valve timing. The rotation phase of the camshaft with respect to the crankshaft is defined as a variable cam timing (VCT) phase hereafter. Actual valve timing (VCT phase) is calculated based on a crank angle signal output from a crank sensor and a cam angle signal output from a cam sensor while the engine is active. The variable valve timing device is controlled in a manner that the actual valve timing coincides with a target valve timing.
Manufacture variation or calculation error of the actual VCT phase is required to be corrected. JP-B2-3395240 describes that a reference phase is learned in an operation state where a VCT phase coincides with the reference phase. For example, the reference phase is represented by a most retard position in an engine idling state. An actual VCT phase (e.g., an advance amount from the reference phase) is calculated using the reference phase as a reference.
Recently, Atkinson cycle is introduced to the engine having the variable valve timing device so as to improve both of gas mileage and engine output, so that a variable range of the VCT phase is made larger. The engine becomes less operated in the operation state where the VCT phase coincides with the reference phase, so that a frequency of learning the reference phase is reduced. It may be possible that the reference phase cannot be learned in the operation time of the engine, depending on a driving pattern of the vehicle. In this case, interval between the learning of the reference phase becomes too much long, and the learning of the reference phase may not be performed for a long time. As a result, accuracy for calculating the actual VCT phase and accuracy for controlling the valve timing may be lowered.